In a typical binary modulation system, the expected receive signal levels may preferably be normalized to -1 volt or +1 volt at the sample time when the received signal decision is made. The decision may be made by taking a slicing level at 0 volts and then deciding whether the received signal is greater than or less than 0 volts at the sample time. This method allows a 1 volt tolerance to signal distortion before an error is made. For example, a -1 volt signal representing a logical 0 would have to deviate by 1 volt to cross the 0 volt threshold and be interpreted as a logical 1.
The received waveform makes transitions between the -1 volt and +1 volt levels in accordance with the data sequence that is sent. If a multiplicity of signal traces are superimposed on a display device such as an oscilloscope, an eye diagram is created. FIG. 1 shows an eye diagram for an ideal waveform while FIG. 2 shows a typical system waveform having inherent distortion commonly introduced by a modem or channel effects. From the eye diagram, or eye pattern as it has sometimes been referred to, experienced operators can derive valuable information relative to the performance of a system. For example, such modem hardware anomalies as filter misalignments, erroneous local oscillation offsets, nonlinearities and demodulator reference errors may be detected. Accordingly, many operational systems include a display device for this purpose. Furthermore, eye diagrams are frequently used in the initial alignment of modems to reduce later field maintenance and adjustment.
The region of the eye diagram where the data decision is made has commonly been referred to as the eye opening. This is the most critical region of the eye diagram as bit errors can be minimized by maximizing the eye opening. Ideally, for the normalized system described, the size of the eye opening is such that there are no traces between -1 volt and +1 volt; this assures a high confidence of data decisions. It is therefore important to provide a means for quantitatively monitoring the eye opening so as to improve performance by improving the bit error rate (BER).